I recently explained to someone why USB before 3.0 must use polling as opposed to interrupts here. Basically, from my understanding of I2C, USB is also a master slave architecture sharing a cheap 1 bit, half-duplex bus, so the devices can't initiate communication or else there will be collisions.
But then, I saw a diagram showing how USB devices are connected on a star shaped network. That seems to imply that the network is switched, meaning the connections aren't hard wired like a bus. This makes sense since loading a bus with 127 devices would make it incredibly slow.
But if it's switched? that means they can avoid collisions by buffering packets. So why not let devices send interrupts?
Best Answer
USB isn't switched in the sense of a peer network of receive-store-forward nodes, like ATM or IP routers. It is more like a circuit-switched network, where the originator (master) specifies the path its wants to use, and subsequently uses it. With USB this path is switched rapidly between all end nodes: polling!